SEISMIC ANISOTROPY ACROSS THE TRANSITION FROM THE RIO GRANDE RIFT TO THE GREAT PLAINS

The evolution of distinct tectonic provinces in the southwestern United States since the Cretaceous, including the Great Plains, the Colorado Plateau, and the Rio Grande Rift (RGR), has been linked to flat subduction of the Farallon plate (~80 Ma) and then its subsequent foundering (~40 Ma). However, there has been a resurgence in tectonic activity (magmatism, extension, and possibly uplift) much more recently (~10 Ma), so there is no clear connection between the Farallon plate’s foundering and present tectonic activity. Small-scale, edge-driven convection is a possible explanation for this renewed activity.

Edge-driven convection, if it is occurring, should extend to the north and south along the margin of the Proterozoic Great Plains craton. Convective flow should have a signature in the upper mantle’s seismic anisotropy and our goal is to determine whether patterns of anisotropy, as determined from SKS splitting that may be consistent with small-scale convection.

Shear wave splitting was measured from the Rio Grande Rift (RGR) to the Great Plains craton to investigate mechanisms of upper mantle anisotropy. Earthquakes recorded at epicentral distances of 90°-130° from EarthScope Transportable Array (TA) and SIEDCAR (XR 2008-10) broadband seismic stations were examined comprehensively to determine whether SKS and SKKS phases indicated anisotropic properties.

Fast polarization directions near the Rio Grande Rift tend to be sub-parallel to the RGR but then change to angles that are more consistent with North America’s average plate motion, to the east. The area above a mantle high velocity anomaly discovered separately, which may indicate thickened lithosphere, corresponds to unusually large delay times and fast polarization directions that are more closely aligned to a north-south orientation. The area of southeastern New Mexico that falls between the mantle fast anomaly and the Great Plains craton displays dramatically smaller delay times, as well as changes in fast axis directions toward the northeast. Changes in fast axis directions may indicate flow around the mantle anomaly; small delay times could indicate vertical or attenuated flow.